1. Field of Invention
The present invention relates to security tags and, more particularly, to a process for making an electrical circuit for use in a security tag.
2. Description of Related Art
Security tags are tags that are adapted to reflect electromagnetic energy in order to indicate their presence within a detection zone. They can be associated with an item in order to monitor the item. Two common types of security tags are resonant inductor/capacitor (LC) circuit based tags and dipole antenna based tags. Both of these types of tags respond to an electromagnetic scanning signal by providing a response signal. The response signal is detectable by suitable signal detection equipment for indicating the presence of a security tag within a scanned detection region or interrogation zone (sometimes referred to as an “interrogator”). In particular, the tag provides a response signal when stimulated by the electromagnetic field at a predetermined tag frequency. A disturbance of the electromagnetic field caused by the response signal is detectable by the signal detection equipment that is tuned to a predetermined detection frequency and is located in the detection region or zone. The signal detection equipment can be adapted to provide an alarm when an un-deactivated security tag is detected, such as commonly accomplished in Electronic Article Surveillance (EAS) applications.
LC Security Tags
LC resonant tags commonly operate in the RF range. The LC circuits of such tags provide a response signal by resonating in response to the electromagnetic energy applied to them at their resonant frequency. In order to detect the presence of an LC based tag in a detection region or zone, the frequency of the electromagnetic energy applied to that region or zone is swept through a range of frequencies that includes the predetermined tag frequency. The LC circuit of the tag resonates when the swept frequency of the applied energy reaches the predetermined tag frequency. A security tag of this type is disclosed in U.S. Pat. No. 5,861,809, entitled “Deactivateable Resonant Circuit,” issued on Jan. 19, 1999 to Eckstein, et al. (Eckstein).
Typically, the LC circuits of LC-based resonant tags are generally planar circuits formed of conductor layers and dielectric layers. One of the conductor layers includes one plate of a capacitor and a spiral conductor coil forming an inductor disposed upon a surface of a dielectric layer. One plate of the capacitor is connected to a proximal end of the coil. A second conductor layer is formed on the opposing surface of the substrate to serve as the second plate of the capacitor. The substrate thus serves as the dielectric of the capacitor. A through connection between the second plate and the distal end of the coil completes the fabrication of the inductor/capacitor (LC) resonant circuit. The two conductor layers can be formed using well known photo-etching techniques. Alternately, the conductor layers can be formed by laser cutting or arc cutting techniques as disclosed in U.S. Pat. No. 5,920,290, entitled “Resonant Tag Labels and Method of Making the Same,” issued to McDonough on Jul. 6, 1999.
Other patents disclosing similar technology include U.S. Pat. Nos. 6,214,444, 6,383,616 and 6,458,465 assigned to Kabushiki Kaisha Miyake (Miyake) which teach a method for making resonant tags in which a circuit-like metallic foil pattern was adhered to a dielectric film prepared from a liquid resin by a coating process. A circuit-like metal foil pattern on one side of the dielectric film is aligned with a circuit-like pattern on the other side of the dielectric film so as to form a capacitor. The dielectric film had openings configured similarly to and aligned with openings in the circuit-like metal foil, wherein the configuration of the circuit-like metal foil pattern and the dielectric film was generally spiral in configuration.
U.S. Pat. No. 6,618,939 and Publication No. US 2004/0025324, also assigned to Miyake, teach a method for making resonant tags wherein a metal foil having a thermal adhesive applied to at least one face is stamped out into a circuit-like shape and adhered to a base sheet. The metal foil is stamped onto a metal foil portion having a predetermined shape while being passed through a die roll having a stamping blade with a predetermined shape. A transfer roll is in contact with the die roll to function as a die back-up roll and to hold the metal foil portion obtained by the stamping operation onto the surface of the transfer roll by suction holes formed in the transfer roll. The stamped out metal foil portion is thermally adhered to the base sheet in contact with the transfer roll by an adhesive roll in contact with the transfer foil through the base sheet.
Another patent assigned to Miyake, U.S. Pat. No. 5,645,932, teaches a method for making resonant tags in which a laminate was fabricated by adhering a metal foil coated with hot-melt adhesive resin film to a carrier sheet such as paper. The metal foil of the laminate was stamped out using a stamping die to provide a predetermined circuit-like pattern. The metallic foil side of the laminate was superposed on a support such as a plastic film. The circuit-like metallic foil was then transferred to the surface of the support by heating the circuit-like pattern from the support side of the carrier sheet side.
U.S. Pat. No. 4,730,095 (the '095 patent), assigned to Durgo A G, teaches a method of producing a plurality of equal printed circuits on a common, planar insulating carrier having an electrically conducting layer on at least one of its surfaces. The electrical circuits have a spirally arranged conductor trace forming at least one induction coil and at least one capacitor.
In the '095 patent, a plurality of reference perforations are applied to the insulating carrier using a laser and a conducting layer is applied to at least one side of the carrier. A portion of the conductive layer having the rough contours of a circuit element is removed. The circuit element can be an inductive coil and the remaining portion of the conductive layer can have a shape and size approximating the outside dimensions of the coil. Computer controlled lasers are then used to remove further portions of the conductive layer to provide conductive tracks which form the electrical circuit. The electrical values of the circuit are determined and compared with design values. The electrical values can be corrected using the lasers if necessary.
U.S. Pat. No. 4,900,386, also assigned to Durgo A G, teaches a method for producing labels incorporating electrical oscillating circuits wherein parts of the circuits are initially punched out of a center area of a metal web covered by an adhesive. The center area is then covered by an insulating material web for handling stability in order to punch out the part of the circuit to be located at the outer web area. A covering foil is laminated onto the metal web and the parts of the Cortez to be located on the reverse side are applied onto the insulating material web and connected electrical to the remainder of the circuit.
This method of fabricating the elements of an LC-based tag has several problems. One particularly significant problem is the cost of the substrate itself and the design limitations placed on the tag by various substrate requirements. Since the substrate is a structural element that must provide most of the structural integrity of the tag, there are minimum requirements on the mechanical strength of the materials that can be used to form the substrate. This limits the number of different kinds of materials that can be used to form substrates. U.S. Pat. No. 5,142,270, entitled “Stabilized Resonant Tag Circuit and Deactivator,” issued to Appalucci et al. on Aug. 25, 1992, discloses selected considerations with respect to substrate strength.
Additionally, the requirement that the substrate provide sufficient mechanical strength to the response circuit imposes a requirement that the substrate be formed with a minimum thickness. This limits the amount of capacitance that can be provided on a unit area of substrate surface. U.S. Pat. No. 5,682,814, entitled “Apparatus for Manufacturing Resonant Tag,” issued to Imaichi, et al. on Nov. 4, 1997, discloses the relationship between dielectric thickness and capacitance. The material of the substrate must also be capable of withstanding the photo-etch baths required to form the elements of the LC circuit. This factor places additional limitations on the materials that can be used in the design of substrates.
Under these circumstances, it may not be possible to optimize the dielectric properties of the substrate when selecting a dielectric material or a dielectric thickness for use as a component of a security tag. The inability to optimize the dielectric properties of the dielectric materials results in many problems, such as increased capacitor size, lower tag yields and hence, increased costs for the fabrication of security tags.
Other problems encountered in forming the elements of an LC-based tag arise from the photo-etching process. For example, the photo-etching process can be slow and quite expensive. An example of a system attempting to obtain high speed printing of security tags using a photo-etch process is U.S. Pat. No. 3,913,219, entitled “Planar Circuit Fabrication Process,” issued to Lichtblau on Oct. 25, 1975. Fine tuning of the capacitance within an LC-based tag, by adjusting the amount of conductive material forming a capacitor plate after the initial fabrication step thereof, is disclosed in U.S. Pat. No. 4,369,557, entitled “Process for Fabricating Resonant Tag Circuit Construction,” issued to Vandebult on Jan. 25, 1983.
In addition to the high cost of the photo-etching process itself, the fact that the process requires environmentally unsafe chemicals creates disposal problems for the spent materials. As will be appreciated by those skilled in the art, the procedures required to safely dispose of spent photo-etching materials significantly increase the costs of producing security tags. Furthermore, substantial amounts of conductive material must be removed by the etching process when forming the conductor layers of the tag. This further increases the costs of the fabrication process as a result of the waste of conductive material and/or the complications of performing various recovery processes, such as recovering aluminum, when forming the tags.
An additional area of difficulty encountered when using the prior art methods for forming security tags is accurate control of the amount of the capacitance in the tags. Inaccurate capacitance can result from variations in the dielectric constant, variations in the thickness of the dielectric material and variations in the alignment of the capacitor plates. The dielectric constant of the material can normally be specified and accurately provided for the materials used in the fabrication of tags. Additionally, the dielectric constant of a material can be tested prior to the fabrication process. Furthermore, the thickness of the dielectric material can normally be controlled by conventional coating technology and tested prior to the fabrication process.
Thus, the most common problem in accurately controlling the capacitance is the alignment of the circuit elements making up the tag. For example, when the second plate of the capacitor is disposed on the second surface of the substrate or over the first plate, much care must be taken to make certain that the second plate is correctly aligned with the first plate. Failure to align the plates correctly results in inaccuracies in the amount of capacitance produced since the actual area of overlap of the plates determines the capacitance. This causes inaccuracies in the frequency at which the tag resonates. Often this results in an upward shift in resonant frequency.
This problem can limit the speed of the fabrication process, increase the costs of the fabrication equipment and significantly lower the yield of the tag fabrication process, for example, by causing tolerance buildup quality control issues in the fabrication process. Furthermore, it is the nature of the capacitor structures formed during the tag fabrication process that small amounts of plate misalignment produce large variations in the capacitance produced and concomitant large variations in the resonant frequency of the resulting tags. This problem tends to be worse for stamped circuits than for etched circuits due to the nature of the substrate and dielectrics involved in the processes. Another problem is that when foil is die cut into a pattern the shearing action may create beveled geometry rather than a planar geometry near the edge of the cut. That is, the shearing action used to cut the foil may create sharp edges on the foil that may cut into the substrate thereby altering capacitance.
Dipole Security Tags
Dipole-based security tags are adapted for operation in the UHF range. The dipole making up such a security tag basically comprises one or more conductive strips, or stubs, that function as an antenna for receiving energy from an applied electromagnetic field. When the received field energy has a predetermined dipole frequency the antenna applies the energy to an associated system (e.g., circuitry) in the security tag to energize that circuitry. The circuitry energized in this manner can be an integrated circuit chip that is wire bonded to the conductive, dipole strips. U.S. Pat. No. 5,708,419, entitled “Method of Wire Bonding an Integrated Circuit to an Ultraflexible Substrate,” issued to Isaacson et al. on Jan. 13, 1998, discloses the use of antenna to energize a system at a predetermined tag frequency that is primarily dependent on the antenna length.
When the circuitry within a dipole-based security tag is energized by way of the dipole antenna, the circuitry responds by providing a reflected signal. The reflected signal from the security tag is transmitted by the antenna thereby disturbing the applied field. Thus, a dipole-based security tag in a detection region can be detected by sweeping the frequency of the electromagnetic energy applied to the region through a range of frequencies that includes the predetermined dipole frequency Suitable detection equipment detects the disturbance of the field when the frequency of the applied energy reaches the predetermined dipole frequency.
It is known to fabricate dipoles for security tags from copper and silver. For example, U.S. Pat. No. 6,375,780 entitled, “Method of Manufacturing an Enclosed Transceiver,” issued to Tuttle on Apr. 23, 2002, teaches forming security tag dipoles from copper and silver ink. U.S. Pat. No. 5,280,286, entitled “Surveillance and Identification System Antennas,” issued to Williamson on Jan. 18, 1994, teaches etching copper foil to form security tag dipoles. However, the use of copper and silver for security tag dipoles is very expensive.
Security tags can be used in many applications. In one of many examples, security tags can be attached to an item sold in a retail sales establishment to monitor the location of item and deter theft. In the retail establishment application, equipment, e.g., a transmitter, for applying an electromagnetic field to a detection region and detection equipment, e.g., a receiver, for detecting disturbances of the field caused by the presence of security tags can be located at or around points of exit from the establishment. Such transmitters and receivers can be combined into a single unit, sometimes referred to as an “interrogator.” Additionally, detection equipment for security tags in retail establishments can be disposed in many other locations on the premises in order to monitor movement of the item within the establishment. Security tags are especially useful in cases where very large numbers of items must be monitored.
In another example, security tags can be attached to an inventoried item in a warehouse or an item being shipped from one location to another in commerce. The use of a security tag in this manner can be especially useful in providing inventory control for very large numbers of items. The use of security tags for inventory control is disclosed in U.S. Pat. No. 6,195,006, entitled “Inventory System Using Articles with RFID Tags,” issued to Bowers et al. on Feb. 27, 2001. Furthermore, security tags can be attached to books, periodicals, audio tapes and like items located in libraries and other institutions that make such items available for access by the public.
Many methods for attaching a security tag to an item are known. One method is to clip a security tag to the material of the item to be monitored. The security tag can also be adhered to the material of the item to be monitored. Additionally, the tag can be clipped onto or adhered to materials associated with the item to be monitored, such as packaging, advertising or informational materials. However, all of the known methods for attaching a security tag to an item are costly and error-prone. The costs of these methods must be borne by the retailers and/or the providers of the goods or services. These costs are in addition to whatever costs are incurred in packaging, identifying or maintaining the items, and providing the required promotional or informational materials for the items.
Many LC security tags must be activated when they are ready for use. Furthermore, they must be deactivated when a sale of an item is consummated or when they are legitimately removed from an item. For example, an LC security tag which is not removed from an item or deactivated at a point of sale in one establishment may set off an alarm from detection equipment located at a second establishment. This can result in an innocent customer being questioned by personnel at the second establishment.
In general, LC security tags are activated and deactivated by shifting their resonant frequency into and out of the frequency range to which the detection equipment is tuned. The resonant frequency can be shifted for the purpose of activation and deactivation by changing the amount of capacitance in the resonant circuits of the tag. U.S. Pat. No. 6,025,780, entitled “RFID Tags Which Are Virtually Activated And/or Deactivated and Apparatus and Methods of Using Same in an Electronic Security System,” issued to Bowers on Feb. 15, 2002 discloses such a system. Another system for shifting resonant frequencies in this manner is disclosed in U.S. Pat. No. 5,103,210, entitled “Activatable/Deactivatable Security Tag for Use with an Electronic Security System,” issued to Rode on Apr. 7, 1992. Additionally, U.S. Pat. No. 4,876,555, issued to Durgo A G, teaches a method for carrying out deactivation using a continuous hole which can be formed by means of a needle roll and is disposed in the insulating layer of a resonant label in the region between two conducting layers.
One method for changing the amount of capacitance in a security tag involves creating a weakened area between the two plates of a capacitor during the tag fabrication process. The weakened area creates a higher electromagnetic field in its vicinity when electromagnetic energy is applied to the tag at the predetermined frequency. U.S. Pat. No. 5,861,809 (Eckstein) discloses another method for changing the frequency in a security tag. An inductor taught in this patent is formed with a discontinuity, or gap, causing an electrical open circuit. The open circuit is closed with a fuse which is secured near the gap and wire bonded to portions of the inductor near the gap. The fuse is melted by a current greater than a predetermined level flowing through it in order to deactivate the tag. A current level which is high enough to melt the fuse can be induced by an external electromagnetic field. Melting of the fuse causes an open circuit condition, which alters the resonant frequency of the tag.
In another example of changing capacitance to alter the resonant frequency of a security tag, one of the capacitor plates can be formed with a dimple protruding from its surface. The dimple provides a shorter distance between the tip of the dimple and the opposing plate, than between the remaining surfaces of the two plates. When a high level of electromagnetic energy is applied to the tag, a voltage in excess of the breakdown voltage can be created between the tip of the dimple and the opposing plate. This causes the dielectric material to break down, thereby substantially short circuiting the two plates to each other. When the capacitor shorts out in the weakened area, its capacitance goes substantially to zero and the resonant frequency of the tag is moved out of the range of frequencies being swept by the detection equipment. Such a dimple for deactivating a resonant tag is disclosed in U.S. Pat. No. 5,142,270, entitled “Stabilized Resonant Tag Circuit and Deactivator,” issued to Appalucci et al. on Jul. 8, 1992.
One problem with the known methods for deactivating tags is that a tag may spontaneously reactivate at a later time. It is believed that one reason why tags reactivate may be that the short circuit between the plates of the capacitor is formed by fragile dendritic structures created by the breakdown of the dielectric. The structures providing the short circuit between the plates can therefore break at a later time, for example, due to flexing of the tag, and restore the high resistance path between the plates. When this occurs, a security tag that is deactivated after a legitimate purchase can set off an alarm if an innocent bearer of the tag inadvertently brings it into a detection region.
It is sometimes desirable to activate or deactivate a large number of security tags at the same time using bulk activation or bulk deactivation techniques. For example, a manufacturer of security tags can manufacture a large number of activated tags. If a container of the activated tags is sold to a retail establishment that is not using a corresponding detection system, they must be deactivated. In another example, an entire container of items having individual security tags can be legitimately purchased at the same time. It is not uncommon for such containers to have dimensions of four feet by eight feet. In each case, large numbers of tags at varying distances and orientations must be activated or deactivated at the same time. Thus, problems may occur when activation or deactivation energy is applied in these examples and tags may not be effectively processed.
Additional references pertinent to the field of security tags include: U.S. Pat. Nos. 4,215,342; 4,260,990; 4,356,477; 4,429,302; 4,498,076; 4,560,445; 4,567,473; 5,108,822; 5,119,070; 5,142,270; 5,142,292; 5,201,988; 5,218,189; 5,241,299; 5,300,922; 5,442,334; 5,447,779; 5,463,376; 5,510,770; 5,589,251; 5,660,663; 5,682,814; 5,695,860; 5,751,256; 5,841,350; 5,861,809; 5,864,301; 5,877,728; 5,902,437; 5,920,290; 5,926,093; 5,955,950; 5,959,531; 6,025,780; 6,031,458; 6,034,604; 6,072,383; 6,087,940; 6,089,453; 6,166,706; 6,208,235; 6,214,444; 6,304,169; 6,458,465; 6,618,939. All references cited herein are incorporated herein by reference in their entireties.